Hillary Clinton, the front-runner in the race for the Democratic presidential nomination, turned down on Monday an invitation by Fox News to debate rival Bernie Sanders in California, despite having agreed previously to a debate in May.

“We believe that Hillary Clinton’s time is best spent campaigning and meeting directly with voters across California and preparing for a general election campaign that will ensure the White House remains in Democratic hands,” her campaign said in a statement.

Sanders had already agreed to participate in the event to be hosted by Fox News, following a series of heated exchanges during their fifth debate in April in New York.

“I am disappointed but not surprised by Secretary Clinton’s unwillingness to debate before the largest and most important primary in the presidential nominating process,” Sanders said in a statement.

The U.S. senator from Vermont has remained steadfast in his long-shot battle with Clinton for the Democratic nomination for the Nov. 8 presidential election, although he lags her in the delegate count with only a few state contests remaining.“Naturally, Fox News is disappointed that Secretary Clinton has declined our debate invitation,” Bill Sammon, Fox News vice president and Washington managing editor, said in a statement, “especially given that the race is still contested and she had previously agreed to a final debate before the California primary.”